Acetic acid is produced industrially by carbonylating methanol in the presence of water, a rhodium catalyst, a metal iodide, and methyl iodide. For the methanol carbonylation reaction, the reaction mixture contains small amounts of by-products (impurities), for example, a carbonyl compound (e.g., acetaldehyde, butyraldehyde, crotonaldehyde, 2-ethylcrotonaldehyde, and an aldol condensation product thereof), an organic iodide (e.g., a C2-12alkyl iodide such as ethyl iodide, butyl iodide, or hexyl iodide), and others. These impurities result in low quality of product acetic acid. For example, a permanganate reducing compound test (permanganate time) detects extremely small amounts of impurities (permanganate reducing compounds; PRC's) even if the extremely small amounts are difficult to determine quantitatively by current advanced instrumental analysis. Unfortunately, acetaldehyde and methyl iodide have close boiling points to each other, and thus it is difficult to separate acetaldehyde and methyl iodide from each other efficiently by an ordinary distillation means alone. Moreover, separation of the impurities by combination of distillation with water extraction has also been reported. According to this process, for coexistence of methyl acetate with PRC's as well as methyl iodide, methyl acetate is dissolved in and distributed to an aqueous phase in the water extraction, and thus methyl iodide may also undesirably be extracted into the aqueous phase. This results in a loss of methyl iodide.
Japanese Patent Application Laid-Open Publication No. 8-67650 (JP-8-67650A, Patent Document 1) discloses a process for removing acetaldehyde, comprising the steps of: separating a reaction mixture of methanol carbonylation into a volatile phase containing acetic acid, methyl acetate and methyl iodide and a less-volatile phase containing a rhodium catalyst; distilling the volatile phase to form a product mixture containing acetic acid and an overhead containing methyl acetate and methyl iodide; separating the overhead into a lower phase (a methyl iodide phase) and an upper phase (an aqueous phase containing acetaldehyde); distilling the lower phase and/or the upper phase in a distillation column (an acetaldehyde removing column) to form an acetaldehyde concentrate from a top of the column; and subjecting the acetaldehyde concentrate to a water extraction.
However, in distilling the upper phase (aqueous phase) containing acetaldehyde, it is necessary to provide a large amount of energy for distillation and separation of acetaldehyde due to distillation of water having a large latent heat for evaporation, or it is necessary to reduce the amount of energy required for the distillation by increasing the number of distillation stages (or plates). Whereas, in distilling the lower phase (methyl iodide phase), it is necessary to increase a reflux amount or to increase the number of distillation stages, due to a small difference in boiling point between methyl iodide and acetaldehyde. Moreover, the distillation of a mixture or homogeneous liquid of the upper phase and the lower phase also involves an increase in the amount of vapor (the amount of heat energy) in the distillation column and/or an increase in the number of distillation stages. This results in economically low production of acetic acid.
Furthermore, the process described in Patent Document 1 fails to increase an acetaldehyde removal efficiency in the distillation column, because acetaldehyde is not concentrated in the overhead effectively.
WO 2014/031407 (Patent Document 2) discloses a process for producing acetic acid, the process comprising the steps of: separating a crude acetic acid composition in a light ends column (a splitter column) into an overhead stream comprising methyl iodide, water, methyl acetate, and permanganate reducing compounds (PRC's), and an acetic acid product stream; separating a portion of the overhead stream in a first distillation column to form a stream enriched in at least one PRC, wherein the enriched stream further comprises at least some of the methyl iodide; and extractive distilling the enriched stream with an extractive agent (e.g., water) in a second distillation column to form a distillate comprising methyl iodide and a residue comprising at least one PRC and optionally less than 1 wt. % methyl iodide. This document also discloses a mass flow ratio of the enriched stream relative to the extractive agent of at least 0.01:1.
Unfortunately, according to this process, for distillation and separation in the first distillation column, as the same as the process described in Patent Document 1, it is necessary to provide a large amount of energy or it is necessary to increase the number of distillation stages. In addition, the extractive distillation of the PRC's in the second distillation column needs a large amount of an extractive agent and a large number of the distillation stages and thus requires a large amount of separation energy. Further, methyl acetate or acetic acid coexistent with the PRC's in the second extractive distillation step is dissolved in an aqueous phase in the water extractive distillation, and thus methyl iodide may undesirably be extracted into the aqueous phase. This results in a loss of methyl iodide.